Drug addiction is considered as a disease perturbing the brain's natural reward system. It is well established that drugs of abuse exert their effects by interfering with dopaminergic neurotransmission. It has been shown for example that the dopaminergic system plays an important role in reward-related behaviours. The major goal of this Project is to determine by which mechanisms DARPP-32, a crucial phosphoprotein integrating upstream dopaminergic signals and regulated by casein kinase 1 (CK1) and casein kinase 2 (CK2), mediates the effects of drugs of abuse and psychostimulants. Using phospho-specific DARPP-32 antibodies and DARPP-32 phospho-site mutant mice, we have demonstrated that changes in the phosphorylation state of DARPP-32 occur in response to the in vivo administration of drugs of abuse. We have identified four regulatory phosphorylation sites converting DARPP-32 either into a potent inhibitor of a major protein phosphatase (PP-1), or into a potent inhibitor of a crucial kinase (PKA) depending of the combination of sites phosphorylated. Two of these sites are targeted by CK1 and CK2. The striatum, the major brain region for the dopaminergic functions, is composed of two important heterogeneous neuronal cell populations. The importance of these two neuronal cell populations for the above mentioned observations is unknown. We will thus extend our previous in vivo studies to include cell-type specific studies of DARPP-32 phosphorylation. In addition, chronic exposure to drugs of abuse has been shown to induce changes in gene expression that are involved in the rewarding effects of cocaine. We will perform studies in a cell-type specific manner to determine the relative contribution of each cell-type to these effects. Aim I will be the characterization of cell-type specific effects of acute and chronic administration of drugs of abuse on DARPP-32 phosphorylation and on mRNA translation. We will further investigate the mechanism by which CK1 and CK2 modulate the effects of drugs of abuse through DARPP-32 using genetically modified mice. In addition we will characterizenovel CK1 and CK2 interactingproteins in a search to find putative upstream regulators. Aim II and Aim III will be the characterization of the role of CK1 and CK2 in the actions of drugs of abuse. Taken together, these studies will provide detailed information on DARPP-32 phosphorylation and DARPP-32-mediated transcriptional events in a cell type specific manner, and also on the importance and the regulation of CK1 and CK2 in the actions of drugs of abuse.